Saturable, high affinity and stereo-specific binding sites for benzodiazepines have been demonstrated both in vivo and in vitro in the mammalian central nervous system. The correlations between the binding of a series of benzodiazepines in vitro and their pharmacologic potencies as anxiolytics, anticonvulsants, and muscle relaxants strongly suggests these sites are receptors mediating the therapeutic actions of the benzodiazepines. Recent evidence suggests these receptors may be functionally linked to receptors for GABA (gamma-aminobutyric acid) and a chloride-ionophore. This macromolecular complex may mediate benzodiazepine-sensitive states such as anxiety, muscular tension, and seizure activity. Saturable and high affinity binding sites for imipramine have been demonstrated in membrane preparations from human platelet and human brain. These binding sites are apparently related to the serotonin reuptake site and are distinct from the cholinergic, alpha-adrenergic, and histaminergic receptors. The peripheral (platelet) and central binding sites appear to have similar, if not identical, binding characteristics. There is a significant decrease in the density of these imipramine binding sites in platelets from depressed patients when compared to platelets from age- and sex-matched healthy volunteers.